This invention relates to xerographic apparatus, and more particularly, it relates to the heat and pressure fixing of particulate, thermoplastic toner by direct contact with a heated fusing member.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of electroscopic marking particles, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto in one of various ways, for example, as by heat and pressure.
In order to affix or fuse electroscopic toner material onto a support member by heat and pressure, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky while simultaneously applying pressure. The action causes the toner to flow to some extent into the fibers or pores of support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of thermal energy and pressure for fixing toner images onto a support member is old and well known.
One approach to heat and pressure fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the fuser roll thereby to effect heating of the toner images within the nip. By controlling the heat transferred to the toner, virtually no offset of the toner particles from the copy sheet to the fuser roll is experienced under normal conditions. This is because the heat applied to the surface of the roller is insufficient to raise the temperature of the surface of the roller above the "hot offset" temperature of the toner whereat the toner particles in the image areas of the toner liguify and cause a splitting action in the molten toner resulting in "hot offset." Splitting occurs when the cohesive forces holding the viscous toner mass together are less than the adhesive forces tending to offset it to a contacting surface such as a fuser roll.
Occasionally, however, toner particles will be offset to the fuser roll by an insufficient application of heat to the surface thereof (i.e. "cold" offsetting); by imperfections in the properties of the surface of the roll; or by the toner particles insufficiently adhering to the copy sheet by the electrostatic forces which normally hold them there. In such a case, toner particles may be transferred to the surface of the fuser roll with subsequent transfer to the backup roll during periods of time when no copy paper is in the nip.
Moreover, toner particles can be picked up by the fuser and/or backup roll during fusing of duplex copies or simply from the surroundings of the reproducing apparatus.
One arrangement for minimizing the foregoing problems, particularly that which is commenly referred to as "offsetting," has been to provide a fuser roll with an outer surface or covering of polytetrafluoroethylene, known by the trade name, Teflon to which a release agent such as silicone oil is applied, the thickness of the Teflon being on the order of several mils and the thickness of the oil being less that 1 micron. Silicone based oils, (polydimethylsiloxane), which possess a relatively low surface energy, have been found to be materials that are suitable for use in the heated fuser roll environment where Teflon constitutes the outer surface of the fuser roll. In practice, a thin layer of silicon oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner images carried on the support material. Thus, a low surface energy layer is presented to the toner as it passes through the fuser nip and thereby prevents toner from offsetting to the fuser roll surface.
A fuser roll construction of the type described above is fabricated by applying in any suitable manner a solid layer of abhesive material such as the solid Teflon outer surface or covering of the aforementioned arrangement to a rigid core or substrate.
In attempts to improve at least the perceived quality of the image fused or fixed by a heated roll fuser, such rolls have been provided with conformable surfaces comprising an elastomeric material such as silicone rubber or Viton (trademark of E. I. DuPont for a series of fluoroelastomers based on the copolymer of vinylidene fluoride and hexafluoropropylene). As in the case of the Teflon coated fuser roll release fluids such as silicone based oils have been applied to the surface of the silicone rubber or Viton to both minimize offsetting and to facilitate stripping. See, for example U.S. Pat. No. 3,964,431.
Commerical heat and pressure roll fusers having an outer conformable covering comprising an elastomer such as silicone rubber adhered to a cylindrical core conventionally utilize a core with a wall thickness on the order of 0.5 inch. In order to render the outer elastomeric covering highly conformable, it is made relatively thick. Characteristically, such fusers have a very slow response time (i.e. the time required to raise the surface temperature back to the fusing temperature after fusing of a copy). Consequently, the core temperature must be maintained at at very high temperature for long periods of time, for example, during the standby mode of fuser operation. This requirement of high temperature for prolonged periods of time leads to shortened useful life of the elastomeric outer covering.
Moreover, even though the core is maintained at a relatively high temperature, there are delays in the copy making process due to the slow fuser response time.